The most basic and essential documentation is make it obvious that the capability actually exists. Right now, if I start writing an answer, I can see some icons telling me about formatting options, inserting images, etc, but nothing that tells me I can use ABC.
IMO The next "documentation level" beyond that should be something like this Lilypond Cheat Sheet: http://lilypond.org/doc/v2.16/Documentation/notation/cheat-sheet. Assumes that the user already knows what he/she wants to notate, and give simple examples of how to notate it.
If the "ABC cheat sheet" also has a link to more comprehensive documentation, that is probably enough - but it's important to give a link to what is actually implemented in ABCjs.
If ABC is being used for human communication, it doesn't matter much if Alice uses notation that Bob hasn't seen before, so long as Bob can make a reasonable guess at what it means, or just ignore it. After all, most humans (with the possible exception of Elaine Gould) use that strategy for reading any music notation! But computers don't "use their skill and judgement" in the same way that humans do.